Optical waveguide modulators used in high-speed optical communications, such as those based on waveguide Mach-Zehnder (MZ) interferometric structures, may require active control of their operating conditions, and in particular of their bias voltage that sets the relative phase of interfering light waves in the modulator. The waveguides of the modulator are typically formed in an electro-optic material, for example a suitable semiconductor or LiNbO3, where optical properties of the waveguide may be controlled by applying a voltage. Such a waveguide modulator may be a part of an optical integrated circuit (PIC) implemented in an opto-electronic chip.
Very high speed optical systems may benefit from quadrature modulation formats such as Quadrature Phase-Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM). These types of optical modulation may be realized using a quadrature modulator (QM) that may be implemented using nested MZ interferometric structures. Such structures typically require controlling several bias voltages. For example, a QPSK or QAM optical signal may be generated by splitting light from a suitable light source between two MZ modulators (MZM) driven by an in-phase (I) and a quadrature (Q) components of an electrical signal carrying data, and then combining the resulting I and Q modulated light signals in quadrature, i.e. with a 90°, or π/2 radians (rad), relative phase shift ϕIQ. For example each of the two MZMs of such QM may be modulated in a push-pull manner by a NRZ (non-return-to-zero) binary signal while being biased at their respective null transmission points to produce an optical BPSK modulation. When their outputs are added together in quadrature, i.e. with the relative phase shift ϕIQ=π/2, a QPSK signal (Quaternary phase shift keying) results.
Various schemes for controlling bias set points of an optical IQ modulator have been disclosed. However, many of these schemes require high-bandwidth processing of the control signal which may be difficult or expensive to implement in practice. To ease constraints on the required electronics and improve the accuracy of the control system, bias control systems that use low-speed dither signals have been suggested. However, many of such systems require the detection of the dither frequency and integer multiples of it, which necessitates having a spectrally very clean, harmonic free dithering signal and corresponding dither detection system in order to avoid parasitic offsets. Furthermore, some bias control schemes that rely on low-frequency dither signals may work well for nearly ideal modulators capable of producing a high extinction ratio (ER) output, but less well for those generating low-ER signals. The term “extinction ratio” of an optical modulator conventionally refers to a ratio of ‘ON’ and ‘OFF’ optical power levels at the modulator output, typically expressed in decibels (dBs), when the modulator is operated to effect an ON-OFF keying (OOK) scheme. The ER reflects modulator imperfections, such as power imbalance between modulator arms of an MZM, and as such may be used as a measure of an optical modulator non-ideality for a variety of modulation formats.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for controlling a bias point of an optical waveguide modulator suitable for use in high-speed optical systems.